1) Field of the Invention
The present invention relates to an optical recording medium having a data layer provided on a substrate of an arbitrary thickness and an optical recording/reproducing apparatus therefor, and further relates to an optical recording/reproducing apparatus for a recoding medium having a plurality of data layers.
2) Description of the Prior Art
The techniques of recording and reproducing data with using laser light have been realized in the form of optical disk apparatuses. There are three different operational types of disk apparatuses: a read-only type apparatus, e.g. a popular compact disk player or a laser disk player; a write-once-read-many type apparatus for handling e.g. document files or data files; and a rewritable optical disk apparatus.
The common technique among the apparatuses lies in applying light onto the substrate of an optical disk and detecting the light reflected therefrom for reproduction of signals. Regardless of the diameter of the optical disk or the recording signals to be used in a disk apparatus, each disk substrate is commonly about 1.2 mm in thickness. In the optical system of each optical disk playback device, an objective lens is employed which has a large numerical aperture for convergence of light close to the limit of wavelength diffraction and can focus behind the disk substrate without aberration. Hence, for convergence of light close to the limit of optical diffraction, the substrate preferably has a specific thickness.
A known optical disk is at least 3.5 inches in diameter. However, it is much desired to reduce not only the diameter of the substrate but also the thickness for the purpose of improving the portability and minimizing the installation area of an optical disk recording/reproducing apparatus. To play back a disk having such a thinner substrate, the playback device needs to have a specified objective lens corresponding to the thickness of the substrate from the reason described previously. Then, the problem arises that a conventional optical disk cannot be played back with the arrangement of such a modified optical system.
Also, for the purpose of increasing the memory capacity, multi-layer optical disks have been proposed having a plurality of data layers. One of such multi-layer optical disks disclosed in U.S. Pat. No. 3,999,009 is shown in FIGS. 8(a)-8(b) in which a detachable transparent plate 9 is provided between a recording medium and an objective lens 8 for displacing the focal point of the objective lens for reproduction of data from an arbitrary data layer. More particularly, the focusing is made on an objective lens side one 2a of the two data layers 2a and 2b provided on both sides of a substrate 1 with the absence of the transparent plate 9 as shown in FIG. 8(a) and on the other data layer 2b when the transparent plate 9 is mounted in place as shown in FIG. 8(b). However, the transparent plate 9 is employed only for increasing the focal length of the objective lens 8 and no aberration of the lens 8 is regarded. In fact, even if the convergence of light is established close to the limit of diffraction as shown in FIG. 8(a), the use of the transparent plate 9 causes aberrations due to a difference in the light path distance across the objective lens 8 and the substrate 1, thus increasing the size of the light spot. Because the light spots on the data layers are different in aberration, the recording of data onto the data layers of the optical disk should be carried out by changing the recording density of data in response to the size of each light spot on the corresponding data layer. The disadvantage is that when the number of data layers is increased, the recording density becomes decreased.